1. Technical Field
This invention relates to an enzymatic process for preparing aliphatic polycarbonates and a method of preparation of prepolymers, using cyclic lactones as one reactant.
2. Related Art
Polycarbonates are a particular group of useful polymers. Many polycarbonates can be molded and thermoformed into established products used in large quantities by various market sectors. In many cases, polycarbonate synthesis is accomplished using organometallic catalysts. For example, poly(ether-carbonate) polyols can be synthesized via copolymerization of propylene oxide with carbon dioxide using glycerol-propylene oxide copolymer as an initiator and zinc hexacyanocolbaltate as catalyst. Similar poly(ether-carbonate) polyols can be prepared via polycondensation of diethyl carbonate with aliphatic diol and glycerol-ethylene oxide copolymer using Ti(OBu)4 as a catalyst. Also, aliphatic polycarbonate polyols reportedly can be prepared using condensation copolymerization of ethylene carbonate with trimethylolpropane and aliphatic diols.
Further, various chemical synthetic methods have been employed to prepare aliphatic poly(carbonate-co-esters). For example, synthesis of poly(butylenecarbonate-co-butylenesuccinate) was disclosed using the following two procedures: (a) polycondensation of dimethyl succinate and diphenyl carbonate with 1,4-butanediol using zinc acetate as catalyst, and (b) chain extension of poly(butylene succinate) diol with diphenyl carbonate using zirconium acetylacetonate as catalyst. In addition, to the above polycondensation methods, reports were found on the preparation of aliphatic poly(carbonate-co-esters) via copolymerization of propylene oxide, carbon dioxide and ε-caprolactone, and ring-opening copolymerization of spiroorthocarbonate and ε-caprolactone.
Enzyme-catalyzed polycondensations between dialkyl carbonate or alkylene divinyl dicarbonate and diol has been known to generate various aliphatic polycarbonates. Also, synthesis of aliphatic polycarbonate polyols using enzyme catalysis has been known. Further, copolymerization of alkylene divinyl dicarbonate with aliphatic triols using Novozym 435 as a catalyst to form hydroxylated aliphatic polycarbonates has also been reported. For the latter, monomer feeds studied include: various activated dicarbonates (e.g., trimethylene divinyl dicarbonate, tetramethylene divinyl dicarbonate, and hexamethylene divinyl dicarbonate) and various triols (e.g., glycerol, 1,2,4-butanetriol, and 1,2,6-trihydroxyhexane). In a typical example, copolymerization reactions of 1,2,4-butanetriol with trimethylene divinyl dicarbonate, tetramethylene divinyl dicarbonate, and hexamethylene divinyl dicarbonate were carried out in bulk at 50° C. for 72 h using 1 wt % immobilized Candida antarctica Lipase B (CALB) as a catalyst to form soluble polycarbonates with Mw values of 900, 1 200, and 1 200, respectively. The highest molecular weight (Mw=5 500) for hydroxylated polycarbonate was obtained via polycondensation of 1,2,4-butanetriol and hexamethylene divinyl dicarbonate using 10 wt % Novozym 435 catalyst. Furthermore, the use of activated divinyl carbonate monomers results in a commercially impractical approach because of the high cost of these monomers in addition to their poor chemical stability.
Because organometallic catalysts with low activity are employed, high reaction temperatures (up to 220° C.) are required for these prior art processes. This often causes unwanted side reactions (e.g., alcohol dehydration to form olefins) and leads to low product purity. Furthermore, the use of diphenyl carbonate as a comonomer by chemically-catalyzed polycarbonate polymerization reactions results in toxic phenol as a byproduct. Also, the use of organometallic catalysts results in metal contaminants in products that are likely toxic or may limit the applications that products can be used in.
Accordingly, there is a desire for new and more efficient methods to prepare polycarbonates.